Until recently, Jared Taylor, long one of the country’s most prominent white supremacists, had never supported a presidential candidate.

“There’s been no one worth endorsing,” he said in an interview. “I mean, for heaven’s sake, was John McCain ever going to do anything useful as far as the legitimate interests of whites are concerned?”

But Mr. Taylor believes he has finally found someone who will: Donald J. Trump.

This year, Mr. Taylor’s voice could be heard on robocalls to voters across Iowa and New Hampshire, urging them to support Mr. Trump. “We don’t need Muslims,” he said on the call. “We need smart, educated, white people who will assimilate to our culture.”

Then came Sunday — a banner day for Mr. Trump in the eyes of white-power advocates.

In an early-morning social media post, Mr. Trump approvingly reposted on Twitter a quotation from Benito Mussolini (“It is better to live one day as a lion than 100 years as a sheep”). Then, in an interview on CNN, he refused to condemn the Ku Klux Klan or David Duke, its onetime grand wizard, after Mr. Duke declared his support for Mr. Trump.

After the CNN interview, Mr. Trump pointed to his disavowal of Mr. Duke’s support two days earlier. In an appearance on NBC’s “Today” show on Monday, he blamed a “very bad earpiece” for his equivocation. And a spokeswoman for Mr. Trump, Hope Hicks, said he broadly disavowed all white supremacist groups.

Mr. Duke took no umbrage. “I’ll laugh it off — that’s fine,” he said in an interview on Fox News Radio on Monday evening. “Donald Trump: Do whatever you need to get elected.”

Intentionally or not, Mr. Trump’s remarks are resonating with — and mobilizing — white supremacists, many of whom have traditionally refrained from participating in the political process.

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Jared Taylor at an event last year in St. Petersburg, Russia.CreditAnatoly Maltsev/European Pressphoto Agency

He has their support, whether he wants it or not.

“I’ve never met him, and I cannot read his mind any better than you can,” said Mr. Taylor, 64, the Virginia-based founder of the New Century Foundation and editor of its website, American Renaissance. “But someone who wants to send home all illegal immigrants and at least temporarily ban Muslim immigration is acting in the interest of whites, whether consciously or not.”

It is difficult to quantify the reach of the various white-supremacist websites that are championing Mr. Trump’s cause. Mr. Taylor says American Renaissance attracts about 300,000 unique visitors a month. Another white-power site, Stormfront.org, claims to have the same number of registered users.

But however limited the practical implications of their support may be, the symbolic implications seem clear.

“You can’t help who admires you, but when white supremacists start endorsing you for president, you ought to start asking why,” said Richard Cohen, the president of the Southern Poverty Law Center, which tracks white-power groups.

Mr. Trump is not the first presidential candidate whose efforts to exploit a mood of mistrust and resentment across much of the electorate wound up energizing those in its most racialist corners. In the 1968 presidential race, Gov. George C. Wallace of Alabama, a champion of segregation, won five Southern states in part by appealing to racial fears in a campaign waged against the backdrop of urban riots across America. He was beaten by Richard M. Nixon, who made a subtler appeal to disaffected white voters.

More recently, in 1996, Patrick J. Buchanan assailed illegal immigration as an “invasion,” referred to Mexicans as a group as “Jose,” and elongated Justice Ruth Bader Ginsburg’s name in a way that many critics took as anti-Semitic — saying he was using coded language to excite bigots without alienating mainstream voters.

Now, Mr. Trump is embarked on his own resentment-based campaign, and it is not limited to a single region: His commanding victories in the last three contests came in the Northeast, the South and the West.

Mr. Trump’s support among white supremacists has been building from the day he announced his candidacy, when he characterized Mexican immigrants as “rapists.”

Since then, Mr. Trump — or “the glorious leader,” as one white-power writer is calling him — has only grown more popular with that constituency, which cheered his proposal to ban all Muslim immigration and his since-debunked claim to have seen “thousands and thousands of Muslims” celebrating the terrorist attacks of Sept. 11, 2001, in New Jersey.

Mr. Trump has amplified the messages of some white-power proponents himself: In January, he resent a Twitter message from a site called @WhiteGenocideTm. And he has done the same with statistics on black-on-white crime that were later shown to be false.

Nor was Mr. Trump’s CNN interview on Sunday the first time he had been pressed to repudiate his white supremacist supporters.

In January, when Mr. Trump was questioned about the robocalls made on his behalf in Iowa by white supremacists, including Mr. Taylor, he said that he disapproved of the calls, but that his supporters were animated by a legitimate anger over the violent crimes being committed by “illegal immigrants.”

Mr. Trump’s failure to distance himself more sharply from white-power adherents has been minutely observed in online discussion forums.

The American Freedom Party, a white power group, has a daily hourlong podcast devoted to him. And Mr. Trump will be a frequent topic at American Renaissance’s annual conference in May.

For people on the fringes of the American political right, Mr. Trump’s campaign has held out the promise of a white-power resurgence.

“The march to victory will not be won by Donald Trump in 2016, but this could be the steppingstone we need to then radicalize millions of White working and middle class families to the call to truly begin a struggle for Faith, family and folk,” Matthew Heimbach, co-founder of the Traditionalist Youth Network, wrote on the group’s website in October.